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TCP CHECKSUM CALCULATION
(See also short Description of Internet Checksum).

To calculate TCP checksum a "pseudo header" is added to the TCP header. This includes:

IP Source Address		4 bytes
IP Destination Address		4 bytes
TCP Protocol			2 bytes
TCP Length  			2 bytes

The checksum is calculated over all the octets of the pseudo header, TCP header and data. 
If the data contains an odd number of octets a pad, zero octet is added to the end of data. The pseudo 
header and the pad are not transmitted with the packet. 

In the example code, 
u16 buff[] is an array containing all the octets in the TCP header and data.
u16 len_tcp is the length (number of octets) of the TCP header and data.
BOOL padding is 1 if data has an even number of octets and 0 for an odd number. 
u16 src_addr[4] and u16 dest_addr[4] are the IP source and destination address octets.


/*
**************************************************************************
Function: tcp_sum_calc()
**************************************************************************
Description: 
	Calculate TCP checksum
***************************************************************************
*/

typedef unsigned short u16;
typedef unsigned long u32;

u16 tcp_sum_calc(u16 len_tcp, u16 src_addr[],u16 dest_addr[], BOOL padding, u16 buff[])
{
u16 prot_tcp=6;
u16 padd=0;
u16 word16;
u32 sum;	
	
	// Find out if the length of data is even or odd number. If odd,
	// add a padding byte = 0 at the end of packet
	if (padding&1==1){
		padd=1;
		buff[len_tcp]=0;
	}
	
	//initialize sum to zero
	sum=0;
	
	// make 16 bit words out of every two adjacent 8 bit words and 
	// calculate the sum of all 16 vit words
	for (i=0;i<len_tcp+padd;i=i+2){
		word16 =((buff[i]<<8)&0xFF00)+(buff[i+1]&0xFF);
		sum = sum + (unsigned long)word16;
	}	
	// add the TCP pseudo header which contains:
	// the IP source and destinationn addresses,
	for (i=0;i<4;i=i+2){
		word16 =((src_addr[i]<<8)&0xFF00)+(src_addr[i+1]&0xFF);
		sum=sum+word16;	
	}
	for (i=0;i<4;i=i+2){
		word16 =((dest_addr[i]<<8)&0xFF00)+(dest_addr[i+1]&0xFF);
		sum=sum+word16; 	
	}
	// the protocol number and the length of the TCP packet
	sum = sum + prot_tcp + len_tcp;

	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
    	while (sum>>16)
		sum = (sum & 0xFFFF)+(sum >> 16);
		
	// Take the one's complement of sum
	sum = ~sum;

return ((unsigned short) sum);
}

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